Directional Phonon Suppression Function as a Tool for the Identification of Ultralow Thermal Conductivity Materials

TL;DR

This paper introduces the directional phonon suppression function, a new analytical tool that helps identify nanostructures with ultralow thermal conductivity, advancing thermoelectric material design.

Contribution

It presents the concept of the directional phonon suppression function and demonstrates its application in optimizing nanoporous silicon for minimal thermal conductivity.

Findings

Aligned circular pores significantly reduce thermal conductivity.

Rectangular pores with same porosity further decrease thermal transport.

The method enables systematic thermal conductivity minimization.

Abstract

Boundary-engineering in nanostructures has the potential to dramatically impact the development of materials for high-efficiency conversion of thermal energy directly into electricity. In particular, nanostructuring of semiconductors can lead to strong suppression of heat transport with little degradation of electrical conductivity. Although this combination of material properties is promising for thermoelectric materials, it remains largely unexplored. In this work, we introduce a novel concept, the directional phonon suppression function, to unravel boundary-dominated heat transport in unprecedented detail. Using a combination of density functional theory and the Boltzmann transport equation, we compute this quantity for nanoporous silicon materials. We first compute the thermal conductivity for the case with aligned circular pores, confirming a significant thermal transport degradation with respect to the bulk. Then, by analyzing the information on the directionality of phonon suppression in this system, we identify a new structure of rectangular pores with the same porosity that enables a four-fold decrease in thermal transport with respect to the circular pores. Our results illustrate the utility of the directional phonon suppression function, enabling new avenues for systematic thermal conductivity minimization and potentially accelerating the engineering of next-generation thermoelectric devices.